Stability mechanism of the molten pool in variable polarity plasma arc welding of medium thickness aluminum alloy

A novel model is developed that enables the heat-force on the weld pool to vary synergistically with the welding current and the plasma gas flow rate in variable polarity plasma arc (VPPA) vertical-up welding. The secondary compression effect of the keyhole boundary on heat flux and arc pressure, as well as the heat-force periodic oscillation effect on the weld pool in electrode negative (EN) and electrode positive (EP) phases are considered. The keyhole evolution, temperature field and material flow in the weld pool are all studied. The findings suggest that if the temperature gradient in the molten pool (GT) fluctuates within a particular range, the keyhole expansion’ rate is almost equal to the keyhole contraction’ rate, and the molten pool can be stable. The primary reason for the narrower range of welding parameters as plate thickness increases is that the fluctuation range of GT decreases exponentially as the plate thickness increases. Utilizing the "complementary" approach and reducing the proportion to fine-tune the welding parameters can promote the GT fluctuates within a specific range. This study is beneficial to determining the optimal parameters for the medium-thickness plate.